A New Inverse Charge Constant On-Time (IQCOT) Control for Improvement of Transient Performance in Multiphase Operations
The proposed IQCOT structure (inside the red box) with a two-phase VR is presented, as shown in Fig. 1. As illustrated, the difference between Vc and Isum × Ri is converted into current by using a gm amplifier, and this current is used to charge a capacitor. Then this capacitor voltage (Vramp) is compared with a fixed threshold voltage (VTH) to create pulse frequency fsw. When Vramp touches VTH, the off-time ends and a fixed on-time (Ton) is started. This method is shown in the steady-state part of the waveforms in Fig 2. In case of a large load step-up transient, when Vc-IL × Ri becomes very large, fsw pulses can occur even before the end of the previous Ton time. In that way, the proposed IQCOT control can achieve a natural pulse-overlapping feature between phases (shown in Fig.2), and thus is able to improve the load step-up transient performance. Since, in each phase, one Ton pulse can occur even before the previous Ton pulse finishes, they can merge together to create a longer on-time in each phase, using the proposed Ton generator. Therefore, the proposed IQCOT structure enables the control to meet the high current with a faster transient response by achieving smooth, natural and linear pulse overlapping in high phase count multiphase operation, along with natural and linear Ton extension at the load step-up transient. Test results are shown in Fig 3. With these two features together, the proposed control can significantly reduce the amount of output capacitance on the board, thus reducing the cost and footprint.